NotSerializableException on saving custom class

Question

My two classes are designed too create an array of StudentData objects (name, Date Of Birth, and ID), that include a toString override to print out all variables. It then serializes the array and saves it to a file named studentdata.txt. It should then read the array from the file and reconstruct a new array object from this data, and print out each item in the array to the console.

Am getting this error when attempting to complie...

Exception in thread "main" java.io.NotSerializableException: Student
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.writeArray(Unknown Source)
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.writeObject(Unknown Source)
    at StudentData.main(StudentData.java:38)

Also am not sure how to properly loop through my array and call my toString method to print out to the console, would i be right in assuming I should use a for each loop? Like this?

                 //for (Student s : ???) {
                  //System.out.println(How do I call toString from here?);

My classes

import java.io.*;         //importing input-output files
    class Student
    {


   String name;                     //declaration of variables
 String DOB;
   int id;



   Student(String naam,int idno, String dob)          //Initialising variables to user data
   { 
          name=naam;
          id=idno;
          DOB=dob;

    }

   public String toString() {
       return name+"\t"+id+"\t"+DOB+"\t";
}



}

Number 2

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;

class StudentData                     //main class
{
  public static void main(String args[]) throws IOException                  //exception handling
  {
         System.out.println("Enter the numbers of students:");
         BufferedReader in=new BufferedReader(new InputStreamReader(System.in));
         int n=Integer.parseInt(in.readLine());


         Student[]  S=new Student[n];                      // array of objects declared and defined
         for (int i = 0; i < S.length; i++)       {

               System.out.println("Enter the Details of Student no: "+(i+1));             //reading data form the user
               System.out.println("Name: ");
               String naam=in.readLine();
               System.out.println("ID no: ");
               int idno=Integer.parseInt(in.readLine());
               System.out.println("DOB: ");               
               String dob=(in.readLine());


              S[i]=new Student(naam,idno,dob);                          

              File studentFile = new File("StudentData.txt");
              try {
              FileOutputStream fileOutput = new FileOutputStream(studentFile);
              ObjectOutputStream objectOutput = new ObjectOutputStream(fileOutput);
              objectOutput.writeObject(S);

              S = null;

              FileInputStream fileInput = new FileInputStream(studentFile);
              ObjectInputStream objectInputStream = new ObjectInputStream(fileInput);

                S = (Student[]) objectInputStream.readObject();
            } catch (ClassNotFoundException e) {

                e.printStackTrace();
            }

              //for (Student s : ???) {
                  //System.out.println();
              }

         }








     }

Answer 1

To get rid of the NotSerializableException: Student exception, just make your Student class implement the Serializable interface. (Note that this is a marker interface, so you don't have to implement any methods.)

class Student implements Serializable {
}

---

To loop through the array S of Student objects, try this:

for (Student st : S) {
    System.out.println(st);
}

I'd pick a more descriptive name for the array though (students would be better). If you declare the array as

Student[]  students = new Student[n];

then your for loop can be more readable.

for (Student student : students) {
    System.out.println(student);
}

Note that you don't have to explicitly call the toString method on an object of type Student. Java will do that implicitly for you when you use an object as the parameter to a method that expects a String.